The proposed research will examine the relationship between neighborhood-level and macrolevel factors and individual HIV seroconversion and self-reported risk behaviors among a cohort of injection drug users (IDUs) in Baltimore, MD. As the epidemic of HIV/AIDS among IDUs persists in Baltimore despite a range of prevention interventions targeted to individual injectors, it is necessary to examine more closely potential risk factors at the neighborhood and macro- levels in order to formulate innovative new prevention strategies to complement efforts currently underway. The primary aim of the proposed research is to identify the relative and attributable risks associated with specific macrolevel versus microlevel risk factors for HIV seroconversion among IDUs in Baltimore. The following hypotheses will be tested: 1) Structural constraints (e.g., concentrated disadvantage, residential instability, population density, mixed land use) increase the risk of HIV transmission; 2) Formal social regulation (e.g., community policing, targeted narcotics crackdowns) increases the risk of HIV transmission; 3) Informal social regulation (e.g., collective efficacy) decreases the risk of HIV transmission; and 4) Narcotics price and purity are related to HIV seroconversion risk. The proposed research will involve the design and conduct of a multilevel analysis to investigate the spatial dynamics of HIV transmission among IDUs that are not entirely explained by the demographic and risk behavior characteristics of individuals. This analysis will begin with a descriptive exploration of the geography of narcotics abuse and HIV in Baltimore from 1988 to 2000, making use of data on HIV incidence and individual-level HIV risk behavior among local IDUs from the Johns Hopkins ALIVE cohort study, and over a decade of data on neighborhood crime from the Baltimore City Police Department. This will be followed by a review of recent and on-going qualitative Baltimore neighborhood assessments to obtain measures of informal social control and to achieve an understanding of the risk environment in which the epidemic of HIV/AIDS among injection drug users in has been unfolding. Spatial and temporal patterns of neighborhood crime and disease will be analyzed using SpaceStat and Geographic Information System (GIS) software. Results of this research will contribute to the design of structural interventions to interrupt causal pathways of both drug use and HIV transmission.